Low alpha particle emission electrically-conductive coating

ABSTRACT

An electrically conductive paste providing low alpha particle emission is provided. A resin and conductive particles are mixed, and a curing agent is added. A solvent is subsequently added. The electrically conductive paste including a resin compound is formed by mixing the mixture in a high shear mixer. The electrically conductive paste can be applied to a surface of an article to form a coating, or can be molded into an article. The solvent is evaporated, and the electrically conductive paste is cured to provide a graphite-containing resin compound. The graphite-containing resin compound is electrically conductive, and provides low alpha particle emission at a level suitable for a low alpha particle emissivity coating.

BACKGROUND

The present disclosure relates to a conductive coating material, andparticularly to a graphite-containing conductive coating material and amethod of forming the same.

Low alpha particle emission materials are employed in packagingmaterials. Alpha-particle emission from packaging materials, near theactive layers of the chip, is now measured in single-digits of alphaparticles/khr-cm². Few materials provide alpha particle emission as lowas, or even lower than, 2α/khr-cm².

Measuring the alpha particle activity at this low level is challengingwith the present-day proportional detectors. Recently developedionization detectors employ active signal rejection based on pulse-shapeanalysis and therefore have ultra-low background detection levels. Oneproblem with such detectors is that, for samples that are smaller indimension than the dimension of the anode, the detectors cannotdistinguish alpha particles from the sample tray from the alphaparticles emitted from the sample itself. Thus, a low alpha particleemission electrically-conductive material is desired to coat the sampletray.

SUMMARY

An electrically conductive paste providing an ultra-low alpha particleemission is provided. A resin and conductive particles are mixed to forma mixture, to which a solvent can be subsequently added. Theelectrically conductive paste including a resin compound is formed bymixing the mixture in a high shear mixer. A curing agent is mixed in.The electrically conductive paste can be applied to a surface of anarticle to form a coating, or can be molded onto an article. The solventis evaporated, and the electrically conductive paste is cured to providea conductive-particle-containing resin compound. Theconductive-particle-containing resin compound is electricallyconductive, and provides low alpha particle emission at a level suitablefor coating a semiconductor package or a sample tray in an alphaparticle counter.

According to an aspect of the present disclosure, a method of forming anelectrically conductive material portion is provided. A mixture of aresin and a suspension of conductive particles in a solution is formed.If necessary a solvent and/or surfactant can be added to ensure theconductive particles are well dispersed and to obtain the desiredviscosity. A curing-agent-including mixture is formed by adding a curingagent to the mixture. An electrically conductive paste-containingstructure is formed in a form of a coating or a mold of the electricallyconductive paste. A conductive-particle-containing resin compound isformed by evaporating the solvent from the electrically conductivepaste-containing structure.

According to another aspect of the present disclosure, an article ofmanufacture including an electrically conductive material is provided.The electrically conductive material includes a cross-linked polymer ofa resin in which conductive particles are embedded therein.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a flow chart illustrating the steps of a process for forming aconductive-particle-containing resin compound according to an embodimentof the present disclosure.

FIG. 2 is a cross-sectional view of an alpha particle detector includinga coating of a conductive-particle-containing paste compound on a sampletray according to an embodiment of the present disclosure.

FIG. 3 is a cross-sectional view of a semiconductor substrate includinga coating of a conductive-particle-containing paste compound thereuponaccording to an embodiment of the present disclosure.

FIG. 4 is a cross-sectional view of a semiconductor chip including acoating of a conductive-particle-containing paste compound thereuponaccording to an embodiment of the present disclosure.

FIG. 5 is a graph illustrating the dependency of alpha particleemissivity from 300 mm semiconductor substrates having a coating of agraphite-containing resin compound on the purity of graphite particleswithin the graphite-containing resin compound according to an embodimentof the present disclosure.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to aconductive-particle-containing electrically-conductive coating materialand a method of forming the same. Aspects of the present disclosure arenow described in detail with accompanying figures. It is noted that likereference numerals refer to like elements across different embodiments.The drawings are not necessarily drawn to scale.

Polymer materials are essentially alpha particle free and also are aneffective barrier in blocking movement of alpha particles from a sourceto an area that is sensitive to alpha particle radiation such as thebinary state of a device in modern semiconductor devices. Typically,polymers are electrically insulating except for a small class ofpolymers that are intrinsically electrically conductive polymers (ICP)due to conjugated chemical bonding or pi bonds, which result inelectrons being much more mobile compared to electron pairs that areshared in covalent bonds. The better known ICPs, polyaniline andpolyethylenedioxythiophene, are typically applied as a thin coating thatwould be much less than 50 micrometers thick. Therefore, several layerswould have to be applied to build up to a thickness that is 50micrometers or more. Also, ICPs are expensive compared to most non ICPbased coatings and encapsulants. Common resin chemistries that aresuitable for polymer based coating formulations include epoxies,acrylics, silicones, urethanes and polyimides. Key attributes that arerequired for a polymer resin include inherently low alpha emissivity andcompatibility with a solvent to allow viscosity control for the purposesof coating. For illustration purposes, the invention described hereinwill use a paste resin. However, one skilled in the art will know thatmany different resin chemistries can be used as a basis for aformulation that can be applied as a surface coating.

Paste based coatings can be made electrically conductive by addingelectrically conductive particles to a level that reaches or slightlyexceeds the percolation threshold for a specific particle geometry:spheres, platelets, ellipsoids, filaments or rods.

As used herein, “conductive particles” refers to particles having amaximum lateral dimension in a range from 10 nanometers to 100micrometers and containing a material having a bulk electricalconductivity (as measured in bulk or in an aggregate state for carbonnanotubes) greater than 10⁻¹ S/cm at 20° C.

The conductive particles of the present disclosure may be graphiteparticles, carbon nanotubes, particles of elemental metals, particles ofintermetallic alloys, particles of conductive metal compounds, particlesof ICPs or combinations thereof.

Many conductive materials such as graphite, copper, silver, nickel gold,aluminum, are typically mined from the earth and therefore, could have ahigh level of radioactivity that far exceeds the low levels that are ofinterest. According to embodiments of the present disclosure, onlyconductive materials having low alpha particle emission are employed inorder to provide a low alpha particle emission surface. As used herein,“low” alpha particle emissivity refers to alpha particle emissivity lessthan 2α particles/khr-cm².

Alpha-particle emissivity is given in areal units because alphaparticles emanating from far below the surface are absorbed by thematerial above it. For instance, a 10 million electron-volt (MeV)α-particle has a range of about 70 micrometers in silicon. So any alphaparticles that are emitted below about 70 micrometers from the surfacewill not be detected at the surface.

In order to provide a conductive material having a low alpha particleemissivity, the following method can be employed.

First, high electrical conductivity materials having an electricalconductivity greater than 2.0×10⁴ S/cm at 20° C. are selected. Such highconductivity elemental metals including Al, Ti, Fe, Ni, Cu, Mo, Sn, Ta,W, Pt, Au, intermettalic alloys or conductive metal compounds areconsidered. In one embodiment, the conductive particles are particles ofan elemental metal selected from Al, Ti, Fe, Ni, Cu, Mo, Sn, Ta, W, Pt,and Au; particles of intermettalic alloys of at least two of Al, Ti, Fe,Ni, Cu, Mo, Sn, Ta, W, Pt. and Au; or a conductive metal compoundincluding at least one of Al, Ti, Fe, Ni, Cu, Mo, Sn, Ta, W, Pt, and Au.

Second, a high purity conductive material is obtained and analyzed foralpha particle emissivity. As used herein, a “high purity” materialrefers to a material having a purity of at least two 9's, i.e., amaterial in which the atomic percentage of the target material is atleast 99%.

Third, candidate materials can be tested for their alpha particleemissivity after they are formed into a flat large-area sheet andmeasured using either a proportional or ionization detector.

Last, the high purity conductive material is crushed or otherwisechanged into particles having a maximum size in a range from 10nanometers to 100 micrometers. If the high purity conductive material isprovided as particles, this step may be omitted. In one embodiment, ifthis step is performed, the emissivity of the powder or particles can bedetermined after they are mixed into and form the conductive layer.

In one embodiment the high purity conductive material can be syntheticgraphite.

Synthetic graphite is made by processing petroleum coke and coal tar atextremely high temperatures such as 2,500° C.-3,000° C. Impuritiespresent in the starting raw materials are reduced significantly.

It has been discovered that adding high purity, synthetic graphiteparticles to common paste resin results in a formulation that can beapplied to a thickness that reaches or exceeds 50 micrometers by spincoating, pouring or spray coating. The graphite containing paste coatingadheres well to mineral oxide surfaces such as an aluminum plate orsilicon, or other substrates such as copper; is thick enough to blockthe travel of alpha particles that attempt to escape from the substratematerials such as aluminum and other materials, and is electricallyconductive.

In another embodiment the high purity conductive material can be carbonnanotubes.

Because of their high aspect ratio (length to diameter), the percolationthreshold is reduced so less material is required compared to sphericalparticles.

In yet another embodiment, the high purity conductive material can beparticles of an elemental metal, an intermetallic alloy, or a conductivemetal compound. Exemplary conductive particles in this category include,but are not limited to, particles of Al, Ti, Fe, Ni, Cu, W, Mo, Sn, Ta,Pt, Au.

The overall electrical conductivity of the paste can be varied based onthe intrinsic conductivity of the particles, the geometry and the volumefraction loading.

Referring to FIG. 1, a flow chart illustrates the steps of a process forforming an electrically conductive material according to an embodimentof the present disclosure. The electrically conductive material is aconductive-particle-containing paste compound.

Referring to step 10, a resin and conductive particles in a suspensionare mixed. The resin is mainly a viscous liquid. The viscosity of theresin can be, for example, in a range from 10⁻² Pa·s to 10³ Pa·s.

In one embodiment, the resin can be paste resin. The paste resin caninclude reactive prepolymers that contain epoxide groups and/or polymersthat contain epoxide groups. The paste resin can also be referred to aspolyepoxides. Paste resins may become cross-linked either amongthemselves through catalytic homopolymerisation, or with a wide range ofco-reactants including polyfunctional amines, acids (and acidanhydrides), phenols, alcohols and thiols.

In one embodiment, the paste resin can be selected from a variety ofpaste resins having a relatively low viscosity so that the amount ofsolvent for the suspension may be reduced. In one embodiment, theviscosity of the paste resin can be in a range from 5 Pa·s to 20 Pa·s at25° C. In one embodiment, the viscosity of the paste resin can be in arange from 11 Pa·s to 14 Pa·s at 25° C. In one embodiment, the pasteresin can be a liquid reaction product of epichlorohydrin and bisphenolA such as D.E.R.™ 331 manufactured by The Dow Chemical Company.

The conductive particles in the suspension can be provided by mixingconductive particles with an organic solution. In one embodiment,synthetic graphite formed at an elevated temperature in a range from2,500° C. to 3,000° C. can be employed. In another embodiment, carbonnanotubes can be employed. In yet another embodiment, conductiveparticles having alpha emissivity less than 2α/khr-cm² may be employed.The purity of the conductive particles can be greater than 99.9% inatomic percentage. In one embodiment, the purity of the conductiveparticles can be greater than 99% in atomic percentage. In anotherembodiment, the purity of the conductive particles can be greater than99.999% in atomic percentage.

The size of the conductive particles can be selected such that apredominant portion (i.e., more than 50%) of the conductive particleshas a maximum lateral dimension in a range from 10 nanometers to 100micrometers. In one embodiment, the predominant portion of theconductive particles can have a maximum lateral dimension in a rangefrom 10 micrometers to 80 micrometers.

The amount of the conductive particles can be selected such that thevolume percentage of the conductive particles with respect to a totalvolume of the resin and the conductive particles is in a range from 0.1%to 40%. In order to enable application of the coating, a solvent may berequired to decrease the viscosity. Since the solvent will evaporate offafter application, it is not considered when determining the volumepercent filler. Alternatively, a reactive diluent could also be used tolower viscosity. A reactive diluent will react with the curing resin andtherefore should be included when establishing the volume fraction offiller.

The organic solution can include any solvent provided that the resin issoluble in the organic solvent and the organic solvent can help dispersethe conductive particles. Exemplary organic solution that can beemployed to provide a suspension of conductive particles include, butare not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide(DMF), and dichloroethane (DCE). The conductive particles can be pouredinto the organic solvent, and the mix of the conductive particles andthe organic solvent can be mixed to suspend the conductive particles inthe organic solvent.

In one embodiment, the suspension of the conductive particles in thesolution (i.e., organic solvent) can be provided by pouring theconductive particles into the organic solvent and employing a tipsonicator to disperse the conductive particles within the organicsolvent. The sonicator may be operated for a time duration from 1 minuteto 1 hour. A mixture of a resin and a suspension of conductive particlesin a solution is thus provided.

Optionally, a surfactant may be added to the suspension of theconductive particles in the solution to enhance dispersion of theconductive particles within the solution. Exemplary surfactants that maybe employed for this purpose include, but are not limited to, Tritonx-100, or Octaethylene glycol monododecyl ether.

The resin and the suspension of the conductive particles is subsequentlymixed, for example, by pouring the resin into the suspension and mixingthe resin and the suspension, for example, by shaking or stirring.

Optionally, some of the organic solvent may be evaporated from themixture to increase the density of the conductive particles and resin.In one embodiment, the mixture may be placed in vacuum or under reducedpressure (less than 1 atmospheric pressure) to facilitate evaporation ofthe organic solvent.

Referring to step 20, a curing-agent-including mixture is formed byadding a curing agent to the mixture of the resin, the conductiveparticles, and the organic solvent. A curing agent is a chemical thatcan react with the epoxide groups in paste resin to form a highlycrosslinked, three-dimensional network of chemical bonds. The curingagent can also be referred to as a hardener. The composition of thecuring agent can be selected based on the curing temperature to besubsequently employed. For example, if curing at an elevated temperatureis desired, a curing agent that is cured at the elevated temperature canbe employed, and if curing at room temperature is desired, a curingagent that is cured at room temperature (25° C.) can be employed. Thecuring agent can be selected from commercially available curing agentssuch as various Ancamine® curing agents available from Air Products. Forexample, Ancamine 1922a can provide curing in 2-7 days at roomtemperature or in 2 hours at 100° C. The volume of the curing agentrelative to the volume of resin can be in a range from 0.01 to 0.5,although lesser and greater ratios can also be employed.

Referring to step 30, an electrically conductive paste is formed byadding a solvent to the curing-agent-including mixture. The addedsolvent is an organic solvent that dissolves the resin. In oneembodiment, the added solvent can be the same material as the organicsolvent employed to form a suspension at step 10, i.e., the addedsolvent and the organic solvent in the suspension at step 10 can havethe same composition.

The amount of the added solvent can be determined based on the desiredviscosity of the electrically conductive paste thus formed. In oneembodiment, the viscosity of the electrically conductive paste isselected such that a high shear mixer can homogenize the mixture of theresin, particles and optional solvent. As used herein, a high shearmixer refers to a mixer capable of mixing a fluid having a viscositygreater than 0.2 Pa·s.

Referring to step 40, a high-shear mixer is a mixer that disperses aningredient within a matrix material to homogenize the distribution ofthe ingredient within the matrix material. The high-shear mixer includesa sawtooth type blade or Cowles blade rotating at greater than 2000 RPMin a reservoir containing the materials to be mixed, to provide shear.An electrically conductive paste including a resin compound is formedafter the added solvent and the curing-agent-including mixture are mixedin the high-shear mixer. The conductive particles provide the electricalconductivity in the electrically conductive paste. The conductivity ofthe electrically conductive paste may be controlled by changing thevolume percentage of the conductive particles with respect to the totalvolume of the conductive particles and the resin.

Referring to step 50, an electrically conductive paste-containingstructure can be formed in the form of a coating, or in the form of amold, of the electrically conductive paste. For example, a substrate maybe coated with the electrically conductive paste. Alternately, theelectrically conductive paste may be molded into a shape prior to, orduring, baking.

Referring to step 60, the electrically conductive paste-containingstructure, whether in the form of a coating or in the form of a mold, iscured by inducing cross-linking of the polymers in the resin. Curing ofthe electrically conductive paste-containing structure enhancescross-linking within the electrically conductive paste-containingstructure. In one embodiment, the curing of the electrically conductivepaste-containing structure can be performed at room temperature or withheat to temperatures as high or higher than 60 degrees Celsius.

Optionally, a baking step may be performed before the curing process.During the baking step, the solvent is evaporated from the electricallyconductive paste-containing structure at room temperature or at anelevated temperature. If the baking step is performed at the elevatedtemperature, the elevated temperature may be selected so thatcross-linking of the polymers does not occur, or may be selected so thatcross-linking of the polymers occurs concurrently with the evaporationof the organic solvent. In one embodiment, two different temperaturesmay be employed so that the organic solvent is evaporated at the firsttemperature, and the curing (i.e., cross-linking) of the resin occurs atthe second temperature that is higher than the first temperature.

The organic solvent is removed during the curing of the electricallyconductive paste. The cured material is a conductive-particle-containingresin compound that can suppress alpha particle emission from thesurfaces to which the conductive-particle-containing paste compound isin contact.

Referring to FIG. 2, the electrically conductive paste-containingstructure can be an electrically conductive coating 231 on a sample tray230 in an alpha particle counter 200. In this case, the substrate towhich the electrically conductive paste is applied is the sample tray230. The alpha particle counter can also include a first electrode 210,a second electrode 220, a gas enclosure 240, and components forproviding electrical bias to the first and second electrodes (210, 220)and amplifiers or preamplifiers. After curing of the electricallyconductive paste, an electrically conductive material can be applied tothe sample tray 230, or to an intermediate material that is placed incontact with the sample tray 230 and is present as a surface coating. Inone embodiment, the sample tray 230 forms the second electrode, 220. Inanother embodiment the second electrode is grounded. In yet anotherembodiment, the first electrode is biased with a positive high voltage.

Alpha particle emission from the sample tray 230 can be suppressed bythe coating 231 in contact with the sample tray 230. Thus, theelectrically conductive paste of the present disclosure can allow use ofless expensive metal conductors as base metal for the sample tray orother materials in “low-background” charged particle detectors.

Referring to FIG. 3, the electrically conductive paste-containingstructure can be a conductive coating 301 on a semiconductor substrate300. The semiconductor substrate 300 can include a semiconductor layer,at least one semiconductor device formed on the semiconductor layer, andat least one dielectric material layer and metal interconnect structureembedded therein. The electrically conductive coating 301 is a coatingof a conductive-particle-containing resin compound. After curing of theelectrically conductive paste, an electrically conductive material ispresent as a surface coating on the semiconductor substrate 300. Alphaparticles impinging onto top of the semiconductor substrate 300 can bestopped by the conductive coating.

Referring to FIG. 4, the electrically conductive paste-containingstructure can be a conductive coating 301 on a semiconductor chip 400.The semiconductor chip 400 can be derived from a semiconductor substrateafter formation of semiconductor devices thereupon by dicing thesemiconductor substrate. The semiconductor chip 400 can include asemiconductor layer, at least one semiconductor device formed on thesemiconductor layer, and at least one dielectric material layer andmetal interconnect structure embedded therein. The conductive coating401 is a coating of an electrically conductive-particle-containing resincompound. After curing of the electrically conductive paste, anelectrically conductive material is present as a surface coating on thesemiconductor chip 400. Alpha particles impinging onto the top of thesemiconductor chip 400 can be stopped by the electrically conductivecoating.

Various articles of manufacture including an electrically conductivematerial can be formed employing the method of the present disclosure.The electrically conductive material includes a cross-linked polymer ofa resin in which conductive particles are embedded therein. In oneembodiment, a predominant portion of the conductive particles has amaximum lateral dimension in a range from 100 nanometers to 100micrometers within the cured conductive-particle-containing resincompound. In one embodiment, a volume percentage of conductive particlesin the electrically conductive material can be in a range from 0.1% to40% in the cured conductive-particle-containing resin compound. In oneembodiment, a purity of conductive material in the conductive particlescan be greater than 99% in atomic percentage in theconductive-particle-containing resin compound.

The alpha-particle emissivity of the electricallyconductive-particle-containing resin compound can be affected by thepurity of the conductive particles employed to form theconductive-particle-containing paste compound. FIG. 5 is a graphillustrating the dependency of alpha particle emissivity from 300 mmsemiconductor substrates having a coating of a graphite-containing pastecompound on the purity of graphite particles within thegraphite-containing resin compound. It can be seen that an about 200micrometer thick coating of a conductive graphite-containing resincompound derived from 99.999% pure graphite particles on a 300 mmsemiconductor substrate provides an emissivity of 0.38+/−0.04alphas/khr-cm², while an about 200 micrometer thick coating of aconductive graphite-containing resin compound derived from 99% puregraphite particles on a 300 mm semiconductor substrate provides anemissivity of 16.9+/−0.5 alphas/khr-cm². The difference in theemissivity of alpha particles is due to the impurities present in thegraphite particles. Thus, the alpha particle emissivity of theelectrically conductive paste and the cured conductivegraphite-containing resin compound can be minimized by employing highpurity graphite particles.

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Each of the embodiments described herein canbe implemented individually or in combination with any other embodimentunless expressly stated otherwise or clearly incompatible. Accordingly,the disclosure is intended to encompass all such alternatives,modifications and variations which fall within the scope and spirit ofthe disclosure and the following claims.

1. A method of forming an electrically conductive material portionhaving an alpha particle emissivity less than 2α particles/khr-cm²comprising: forming a first mixture of a resin and a suspension ofconductive particles comprising a conductive material in a solution;forming a second mixture by adding a curing agent to said first mixture;forming an electrically conductive paste by adding a solvent to secondmixture; forming an electrically conductive paste-containing structurein a form of a coating or a mold of said electrically conductive paste;and forming a conductive-particle-containing resin compound byevaporating said solvent from said electrically conductivepaste-containing structure.
 2. The method of claim 1, further comprisingcuring said electrically conductive paste-containing structure toenhance cross-linking within said electrically conductivepaste-containing structure.
 3. The method of claim 2, wherein saidcuring of said electrically conductive paste-containing structure isperformed at a temperature in a range from 0 degree Celsius to 60degrees Celsius.
 4. The method of claim 2, wherein said curing of saidelectrically conductive paste-containing structure is performed at atemperature greater than 60 degrees Celsius.
 5. The method of claim 1,wherein a predominant portion of said conductive particles has a maximumlateral dimension in a range from less than 1 micrometer to 100micrometers.
 6. The method of claim 1, wherein a volume percentage ofconductive particles with respect to a total volume of said resin andsaid conductive particles is in a range from 0.1% to 40%.
 7. The methodof claim 1, wherein purity of said conductive material in saidconductive particles is greater than 99% in atomic percentage.
 8. Themethod of claim 1, further comprising dispersing said conductiveparticles in said suspension by sonicating said suspension prior toforming said first mixture of said resin and said suspension.
 9. Themethod of claim 8, wherein said first mixture is an organic solvent thatdissolves said resin.
 10. The method of claim 9, wherein said organicsolvent is selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide(DMF), and dichloroethane (DCE).
 11. The method of claim 1, furthercomprising adding a surfactant to said suspension of conductiveparticles, wherein said surfactant enhances dispersion of saidconductive particles in said solution.
 12. The method of claim 1,wherein said electrically conductive paste-containing structure is anelectrically conductive coating on a semiconductor substrate or on asemiconductor chip.
 13. The method of claim 1, wherein said electricallyconductive paste-containing structure is an electrically conductivecoating on a sample tray in an alpha particle counter.
 14. The method ofclaim 1, wherein said conductive particles are comprised of graphite.15. The method of claim 1, wherein said conductive particles comprisecarbon nanotubes.
 16. The method of claim 1, wherein said conductiveparticles are particles of an elemental metal selected from Al, Ti, Fe,Ni, Cu, Mo, Sn, Ta, W, Pt, and Au; particles of intermettalic alloys ofat least two of Al, Ti, Fe, Ni, Cu, Mo, Sn, Ta, W, Pt, and Au; or aconductive metal compound including at least one of Al, Ti, Fe, Ni, Cu,W, Mo, Sn, Ta, W, Pt, and Au. 17.-27. (canceled)